Automatic gain control system



sepmo, 1952 A vvvvv v? INVENTOR h//LL/AM /L/EMPH/LL ATTO NEY Patented Sept. 30, v1952 s PATENT NOFFICE l u v Y l 2,612,58 4 y f AUTOMATIC GAIN CONTROL SYSTEM Willam'B, Hemphill, Dallas, Tex.,lrassignor, by"

mesne assignments, to Socony-Vacuum Oil" Company, Incorporated, New York, N. Y.,- 'a corporation ofvNevv York Application November 7, A1946, Serial No. 708,331

SClaims. (Cl. 179-7171) f 1 This invention relates to arrangements for controlling the gain of an amplifier system 'and is particularly concerned With control of the gain of reflection amplifiers used in seismic yprospecting systems. v

I. lngeneral, the gain of an amplifier is varied by rectification of the amplified signal to provide a variable direct-current biasing voltage applied to i ple, of an electronic tube.

one or more'of the amplifier tubes and the ratev rapidity of response of an automatic gain-control systemfis variedlas a predetermined function of time beginningfwith occurrence of a particular event; Vmore particularly and with respect to the amplicationand recording of seismic signals resultinglfrom the detonation of .an explosive charge for reflection from strata below the earths surface, itis .provided that the time lconstant of thegain-.control system shall be initially low so as to insure irrimediat'e'and marked reduction ofthe amplifier gainin response to the iirstorv direct seismic waves .and vit is further provided that in the'followinginterval the time constant of the gain-control system' 'shall be increased in avoidance of markedchanges in amplifier gain so far asadjacent groups of the subsequent signals are concerned butwithout impairment ofthe effectiveness of` :the automatic gain-control system to correct for the general trend of the level of signals due tothe reflected seismic waves. This has the advantage that the gain of the amplifier is high at the ,timevthe' charge is detonated. In consequence, the'signal comprising the rst break, or the v'signal resulting from the direct-traveling Waves, has. a steep wave front as appearing on theseismogram, thus precisely indicating the time of arrival ofA the 'first direct-traveling'wave. However, by'providing that vthe time constant of the gaincontrolsystem shall initially be low, the gain is contracted with great rapidity to take care of these signals Whoseamplitude is .much greater Further and `more, specifically accordance' with the invention,.,the rectifier circuit of the 'binations, and arrangements having feature novelty hereinafter disclosed and claimed AWork I5Y or equivalent having suitable freqH noy- The time'c(instantfof?l the network and therefore the speed of*` response of the automatic gain control is varied by'chanv -f' ing the grid bias of the tube preferably` provided,` for seismic-signal amplifiers, by the combinat of a resistance-reactanceI network and a sou fixed voltage which is connected in circuit"4 the with, in response, to a time-break signalsq'th after to effect increase of the time consti;r the network as apreselected function of time The invention further' resides in systems, Vc

Fora more detailed understanding,ofv ,y `yn` vention and for illustration of a preferredgein bodiment thereof, referencvse is made tothe acc panying drawings, in'which: j n

ll'g.,l is a schematic diagramofanfamplifl channel for a seismicfprospec'tingsystemif' Y Fig. 2 is an explanatory fgurelreferre t ^in discussion of the operation of thesystemjoflig.

In seismic prospecting systems,` V thejseisin waves resulting from explosionjof a vclji'argeja produced below the surfacel'of the earthware ceived by a `spread or groupmofvY geophones .equivalent suitably spaced with respect, tofleach other and toA the explosion point. The. signals produced by thenseveral geophones',I after am'l iication, are impressed uponafsuitab/,le Arecordnigf device, for example, a multiple traceloscillograph'. which produces/afrecordof "the various signals",

which maybe interpreted in determination of the u depth' and `contours of subterranean strata. 'Tou.: produce a, recordinwhichfthe variouswaves' as', received by the several, geophones are clearlydisyf tinguishable, the gain of each o f the amplifiers is" automatically controlled.` l u l V y, I E 'I Referring to Fig.` 1, the signal produced by i.;hqe" geophone l0 is impressed asby `'tran,sforrner upon the input circuit ofA an amplierulz vof suit- 'I able number of stages and which in theparticu; l lar `arrangement shown' includes the'fjan'plien tubes I3 and I4 suitably coupled by. la filter nete,v

response characteristics lto favor frequencies ojol responding withr seismic -wave frequencies `and discriminate against other frequencies.` The'tubelf I4 may befollowed by one or moreAadditiorialuy amplifier-filter stages generically representedQby gain-control system comprises a resistance-reactancevnetworkvvhichlincludes as one ofits com po'nents the anode-cathode resistance, for ex'amlblock I6 and the. output of the amplifier a recorder represented by block l1'.

The rectifier 22, which may be of any suitable type, such as a diode, derives from the signal output of the amplifier I8 a direct-curijeritgaincontrol voltage proportional to the amplitude of the seismic signal and which Viiivthetpartioular system shown is applied tol varythe potential of the suppressor grids of the tubes I3Hand Id t of the signal amplier I2.

the internal or interelectrode resistance of tube 3l is controlled, in the particular arrangement shown, by .variation of its grid potential. The control grid circuit of tube 3| includes two batteries 33 and 34, or other source of direct-current voltage. Assuming that contacts 35 and 36 are open, the potential of thegridvof tube 3|, with respect toground, is thatofasource alone. In considering the total bias on the tube, it is necessary to also consider the potential of the cathode with respect to ground, which, at the arrival oftheftimebreak, is that of source 23 alone. However, since 2 3- is considered to be a xed voltage source, thechange in bias on the tube The rectifier circuit includes,.,the 4battery 23.

or equivalent source of direct current which determines the potential of the suppressorA grids of tubes I3 and I4. and therefore the gain of the amplier, under conditions of zerosignal.

potential or bias is ,Varied by. thealgebraic addition thereto of a variable.'directecurrent biasing. voltage derived by rectifler22froin thev The time constant of resistance-reactance network 24 determines the rateof change of the due-Q A the, Wange of potential of the grid with respect,` to ground. For the purposes herein specicallyydiscussed, the magnitude of voltage 33`may be assumed to be such that the internal resistanceof tube 3| is at or near its minimum value at the arrival of the rst breaks, and consequently. the. tiene.Tv constant ofv network 2li-fis et' onnearl-itaminimum, so providinsior rapidi, ohelle'ev ofI the gain.:control voltage Y derived. from. the output oftheselected .stegeor` aniizller l2..v

Upon closure of contacts 35'and 3.6, current,.-

` ows fromme-source 34 througlrrthe resistor` 1,'

ygain-control.voltage applied Yto the suppressor.

` grids of tubes I3 and I4V for any` given amplitude of, signal ,impressed upon -the rectifierr circuit. Itis' desirable for controlofgai'n of seismic signali ampliers, forv example, that in the interval following a shot, the automatic gain control4 be rapidly. actingl in. order immediately and to marked.v extent to contract the amplification, in responseV to the rst direct-traveling waves due to theshock. Initially the ampliiier gainl is.,` high toinsurethatjhelwave front of tlieiirst` waves-asrecorded by the seismograph shall/.be very4 steepfarid so` afford a precise time indication. and the gain control must be very` L fastracting to. keepthe recorded. traces4 of the slgnalspicked up-by thegeophones .of a spread within.. bounds t avoiding a confused commonV rfol'd.V However, an automatic gain-control systemhaving .high speed of response best suiting it forgthisl .purpose is ill suited .for controlling thegainof, the amplifier to give a good record,

ofthe subsequent reflected waves; for example, for successive Ygroups of such Wavesthere would bemarked change in gain of the amplifier with consequent.. greater difiiculty` in interpretation of the seismogram. Instead of selecting acompi'omise time constantfor network 2 4, it is providedby thevpresent inventionthat initially the time constant ofjthel network, 24 shall be low but. shall be greatly increased, independently of signal; amplitude, forl reception of reflected seismic waves to avoid krriarked diierencein gain.

ofv .the amplication during` the passage therethroughrof adjacent groups of signals, the automatic gaine-.Control system, however,I remaining effectiveV to vary the amplifier gain,l as anl injvelseffunction of signal amplitude,rin aceordance.- with. the general trend of the sienellevel In, attainment-of that, end, the anodercathfide. resistance of the tube 3| isutilized. as one of the resistors ofl the ,Slllflothillg.netiiloiFkVV 24. SpeeieeIly, the,anode-cathoderesstaneeof tube 3|, islefifectively `in .s'hunt lto Athe resistors 2a rid 215er, network; 2.4Lr The. effective, magnitude.. of

thus produeinaeeross thetresistor avczltlase'oral?.v effective tocharge.. the. :condenser 3S; at a.; rate determinedbythemegilitiide of vresistance of ref.

sister. @and the. capacity of 'ecirldeltlser .39,1Y Y The. potential. difference across .condenser 391- .ee'ol tively in series with the voltage of the ,souree y So that. thepoteiital ofthe .eontroljsrid otjtube f progressively lengtiienedwith` consequent reduc-f,

end. condenser.. 39.. Ereterablyy. andes., Shown the. resistor` 138' is variable. ier predetermination.: of the Yrete' et .Whehthetiibe iioteritieljoi grid.' 3J; andtherefore' its anodefeatllode.resistance- It therefore.V appears thetiinon @los teotsend Sztlie time consiente inV the; automatic gain-,control circuit will'jbe'n tion in the'speed @response ofthe control systemzto =harlgeeir1.Y leveloi For example, as showniri Eig, 2, p1lQlI,l7.Q-Gh timeofclosure BVA of contacts 35and'3thetimej leoiostantof `network-2limariee low, Whereas'upon..A

oa Thyratron 4i eoupledas ,by transformant.. n tothe soureeoftheLtime-breals.signal, In zioeordanee.withlueuelipractice,,thelhyratronisfsoL biasedr es. by. Source of. voltage .,43 ytliatrio Eplate., Current flows. until.. arrival ,ofY that. Signal- The -f Switch: 44 is' included to r`break the, ,anodel. Circuit?Y of a Thyratron after the end ofgtlierecording 'period t0 reset the .system for .thetiiextysliota Forzcheneins the time constantotiietiizork14.1 11i response toa, first-break? .signal duetoar-Al rival vatthe Vgeenhone. of, thefdirectf-trayeling Waves instead' ofU to, ottime-breek? sisrial-.-

thetime yconstant'ofr'ietwork 24 in 'response to the' initiating Asignal abruptly and 'by alarge step.

Such action isattainable with th'e arrangement shown byselection o r` adjustment of the magniy tudesof resistor 38 and capacitorf39j-so Vthat the spndin'gly'low.- i

f Itshall be 'understood that thel amplier channels* associated with the vgeophories of A' spread may be all similarlyf'controlled byajsingle'master product of their resistanceand capacityis correcontrollr comprising -fthe -resi'stance-reactance ntwork''lj 38: and "39,: the associated sources fof directfcur'rent .33, V3a and reiayfw. "Briefly stated; the grid oftube 3| may be connectedas'indicated by 'ccndct'or'dto the corresponding grids of tubes corresponding with ktube 3| and similarly associated with the smoothing networks of the automatic gain control systems of otherl amplifier channels. im. f' 1 I:

It shall be underst ood` the inventionis not limited tothe specific arrangement and use v described-but may be usedwin'other relationsv and with changesj and modifications'- within the scope of the 'appendedclaims."" L

.l,`An automatic 'gain-control arrangement for a seismic-signal--amplifier comprising a rectifier responsive' to ,the 1 seismic signal inl said amplifier to supply a direct-current biasing voltage for one or more stages of the amplifier, means for applying said direct-current voltage to said stages including a smoothing network in circuit with said rectifier, an electronic tube whose anodecathode resistance affects thetime constant of said network, and means for varying said resistance to change the speed of response of said gaincontrol system as a predetermined function of time and independently of signal amplitude comprising a resistance-reactance network in the input circuit of said tube, a source of fixed directcurrent voltage, and switching means operable to include said source in circuit with said resistance-reactance network.

2. An automaticfgain-control arrangement for a seismic signal amplifier comprising a rectifier in the output system of a stage of the amplifier to supply a direct-current biasing voltage for one or more stages of the ampliiier, means for applying said biasing voltage to said stages including a smoothing network in circuit with said rectier, an electronic tube whose anode-cathode resistance affects the time constant of said network, and means for varying said resistance to vary the speed of response of said gain-control system comprising a resistance-reactance network in the input system of said tube, a source of xed direct-current voltage, and a time-break relay for including said source in circuit with said resistance-reactance network.

3. An automatic gain-control arrangement for a seismic signal amplifier having such characteristic that the signals due to the initial direct waves effect substantially immediateand marked reduction of amplifier gain whereas the gain is thereafter more slowly Varied to correct for general trends in signal amplitude without marked change for adjacent signal' groups'comprising a rectifier in the output circuit of a stage of the amplifier to provide a biasing voltage for one or more stages thereof, a low time constant network sponsive to a time-break signal for:includni'fsaidf` source in circuit withsaid?resistance-reactarice\y network to effect increase of said "anode-'cathode f resistance l as a predetermined v'function- .of..' time; 4L An" automatic gain-control 'arrangement 'fori 'an amplifier comprising a -rectier in the 'output'vv system of a stage offtheamplierf'to provide a" gain-control voltage for'- one' -or morev stages theref off-a resistance-reactancenetwork interconnecting said rectifier withsaid stages, an'electronic' tube whose anode-cathode resistance is included; in said network, a source-of X'e'd biasing"= vc` 'ltage' in the grid circuit of said tubefordetermining ia source of fixed voltage biasingthegrid'off-sa d the minimum time constant of said network; and? means for increasing the time constant of fsaid network including a -second source offbiasing: v

voltage and a second resistance-reacta'nce' n work in circuit therewith in saidA 'gri'dcircuit 5.?'An automatic gain-control 'arrangement fon a Vseismicl signal amplifier Vcomprising 'a'fietierf in the'output system o'f-'an amplifier-stage tof provide a gain-control*voltage"forone'.` amplierstages, a resistance-reactance invcircuit with said rectifier and comprising ft e anode-cathode'resistanca'of an electronic 'tubl tube to extent insuring' immeaiatend mifrke'i changein"`the magnitude `of said' gain-'cont` l? voltage in response to the seismic signals'first f followingva shot, 'and' means yforl increasing lthev time constant of said network in avoidance of marked changes in gain for adjacent groups of the subsequent signals comprising a second source of biasing voltage, a second resistancereactance network, and means responsive to a time-break signal effectively to include said second source and second network in the grid circuit of said tube. 1

6. A seismic-signal amplifier 'for a seismic prospecting system arranged to have high gain before detonation of the charge, thereafter to have its gain contracted and later expanded with an automatic gain control whose. time rate oi' response is high prior to the shot instant and is made lower after the shot instant which comprises means for varying the amplifier gain comprising a rectier responsive to the amplified signal and connected to apply the direct-current voltage derived from the signal as a biasing voltage to one or more stages of the amplifier for control of gain as an inverse function of signal amplitude, means for varying the-speed of response of said automatic gain-control means to changes in signal level as a predetermined function of time comprising a network in circuit with said rectifier and including a resistance variable to change the time constant of said network,"

and means for varying said resistance'solely in varying in accordance with saidpredetermined y' fimcion-si of-time;andindependentlyr 0f'j Signal amplitude: r

-V'h An-automaticg gain,- control-system` 'for'v a seismic signal amplier comprising, rectii'ying meansgforffderiving1a:directfcurrentbiasing voltagelzwliiclr varies.` in accordance with the. amplitudeaofi: signal amplified. thereby, a resistance-u rcactance;V network: interconnecting said rectifyingarneaans` and: said, amplifier; for applying said voltageztorsaid amplifier, anelectronic tubewhose anodencathode pathgforms. aresistor of said riet--i Work,asecondiresistancefreactance network and a 'sourcezofV direct-currentvv voltage.' both inv thev grid: ciroutxof' said tube-and. external Vto ythe cir cuit including said; rectifyingl meansand. said.

mstnamed .resistance-reactance network, and means; 'operativer ini .coincidence with ,the` occur.- rence. *0iv a` .preselected event -for initiating. lpredetermined.changesA in' the.direct-currentvoltagel oxrthe; ,gridE of said' .tube to eiect progressive variation of the time-constant of said first-named` resistance-reactancef; network independently off the signal amplitude. 81, Anzamplier for a- -sesmic prospecting system; arranged to: havea -high Vgain beforefdetonaf ticnof thefchargegeneratingseismic Waves and. thereafter. to have its gain contracted and laterv expanded with an automatic gain controlwhose time ratefof response is high prior tozthe;detona tiorr'2 of said charge andfis progressively made lower @at f-a `predetermined rate after; detonation cisaid charge which comprises an. automatic gainqcontrol circuit including a rectifierconnected` to-1 derVe--from-.ar seismic wave signal inthzefamplifier again-control voltageappled to at' leastpne stageofthe' axnpiiiier.I ltcvary; the, gain:` as-'faninversefunction of-.signalamplitude,A I ne'fn'is. for-.varying thespeed of responsefofsad gaines control circuit including al resistancef-reactance: networkin said automaticfgain control; circuit:

off said amplier; a;- vacuum tube; whose. inte'r.-` electrodel resistancey forms aportion of said: ref, sistance-reactance network, and .biasing means inA thegrid circuitof. said tube,l and externalto thegain-control circuit including said rectifier to f control said .Y interelectrode resistance.l inde-v pendently of signal amplitude, and means 'operable in predeterminedtime relation withV respect tothe instant. of. detonationof said, chargeto` varyvv said ini',.ere1ectrode.v resistance as ra., predeV termined-functionof time .fthereby to vary the.. time .rateof `response :of said automatic: gain-con@ trol systemindependently ofJ the signalsreceivedby. said.l amp11er.

l WILLrAMBHEr/IPHILL. 'intrecciations errno f The: nfollowing references are `lvof record. in. .the

file of this patent: 

